This type of electrohydraulic automatic transmission, comprising for example three forward ratios with shifting under torque and one reverse ratio, generally utilizes the combination of a power train and a low energy level control-train. The power train which transmits the energy from the drive motor of the vehicle to the driving wheels comprises in particular a hydrokinetic coupling converter whose function is to insure flexible connection between the motor and the transmission mechanism. The power train also comprises a gear box with one or several epicyclic gear trains actuated by a set of gears and hydraulically controlled brakes. In an automatic transmission with three forward ratios, the gear box thus comprises a train of epicyclic gears actuated by two gears and two hydraulically controlled brakes. The gears and the brakes are fed by a hydraulic assembly controlled by electrically actuated valves, two in number in the case of a transmission with three forward ratios; these provide the desired combinations of the pinion assembly, so as automatically to obtain the different speed ratios of the transmission.
The low-energy control, associated with the power train, selectively feeds the electrically actuated valves with signals that are a function of information concerning the running or operation of the vehicle equipped with the transmission and a function of the driver's decisions.
One example of information depending on the operation of the vehicle, is the position of the accelerator pedal, which corresponds to the load placed on the drive motor of the vehicle as well as the speed of the vehicle. The driver's decisions may be represented in particular by the position of a manual control member which allows the imposition of certain transmission ratios and an end-of-the-course contact of the accelerator pedal, a so-called "retrocontact" permitting the transmission ratios to be retrograded by imposition. The actions controlled by the driver act on the automatic system only at certain speed levels of the vehicle so as to insure the mechanic stability of the gear box.
Electronic control devices of this type are already known using as a parameter the speed of the vehicle and utilizing for this purpose a tachometer type of pickup from the automatic gear box. In these known devices, the speed detectors of the vehicle are rotationally connected with any output shaft of the automatic transmission. Reference is made in particular to French Pat. No. 1,397,442 and its additions Nos. 88,608 and 90,531 entitled "Alternator with rotating magnet and adjustable voltage, applicable in particular to power transmission control in automobiles", to French Pat. No. 1,457,550 entitled "control governor as a function of the frequency and of at least one axiliary parameter", as well as to French Pat. No. 2,076,715, entitled "Improvements in governors for automatic gear boxes".
All these known devices have the disadvantage of increasing the space taken up by the automatic transmission and thereby making it difficult to install the motor-propulsion unit, including the transmission in a transverse position, in relation to the longitudinal axis of the vehicle.
An object of the present invention is therefore to replace the speed detector devices of the known type of a direct tap of the speed information of the vehicle onto an existing member of the gear box having on its periphery regularly spaced teeth. It may, for example, be an output pinion of the gear box, or of a parking wheel or of an equivalent member rotating at a speed proportional to the speed of the vehicle. In accordance with the invention, speed detection may then be accomplished by means of an electromagnetic proximity pickup located a short distance from the rotating member the speed of which is to be determined. This arrangement makes it possible to reduce considerably the space required by the automatic transmission by permitting the propulsion/transmission unit to be mounted either longitudinally or transversely in relation to the vehicle and to function equally well in either position.
In the electronic control devices of known type for automatic gear boxes the principal function of the electronic unit is to decide the speed ratios to be engaged, while the function of carrying out the speed ratio changes is left to hydraulic devices. Reference is made in particular to French Pat. No. 1,453,300 entitled "Voltage control device particularly for changing the ratio in an automatic vehicle transmission", No. 1,457,550 entitled "Governor for control as a function of the frequency and at least one axiliary parameter", No. 1,520,662 entitled "Improvement in electric control devices for gear boxes" and No. 2,076,715 entiled "Improvement in governors for automatic gear boxes".